evolution

The evolution of the modern horse has taken place over 50 million years, from the fox-sized four-toed Hyracotherium to the largest draught horse at 17 hands (1.73 m/5.6 ft) at the shoulder.

In evolutionary terms, the period from the appearance of the early hominid Australopithecus 3.5–4.4 million years ago, to the development of modern humans approximately 200,000 years ago, has been fairly short.

A portrait of the English naturalist Charles Darwin, copy by John Collier (1850–1934) in the National Portrait Gallery, London. As the originator of the theory of evolution based on the concept of natural selection, Darwin's publication of his famous treatise The Origin of Species (1859) aroused considerable opposition, mainly from the church, and led to an historic debate at Oxford in 1860 on whether man was descended from ‘apes or angels’.

Slow gradual process of change from one form to another, as in the evolution of the universe from its formation to its present state, or in the evolution of life on Earth. In biology, it is the process by which life has developed by stages from single-celled organisms into the multiplicity of animal and plant life, extinct and existing, that inhabits the Earth. The development of the concept of evolution is usually associated with the English naturalist Charles Darwin who attributed the main role in evolutionary change to natural selection acting on randomly occurring variations. These variations in species are now known to be adaptations produced by spontaneous changes or mutations in the genetic material of organisms. In short, evolution is the change in the genetic makeup of a population of organisms from one generation to another. Evidence shows that many species of organisms do not stay the same over generations. The most dramatic evidence of this comes from fossils.

Evolution occurs via the following processes of natural selection: individual organisms within a particular species may show a wide range of variation because of differences in their genes; predation, disease, and competition cause individuals to die; individuals with characteristics most suited to the environment are more likely to survive and breed successfully; and the genes that have enabled these individuals to survive are then passed on to the next generation, and if the environment is changing, the result is that some genes are more abundant in the next generation and the organism has evolved.

Evolutionary change can be slow, as shown in part of the fossil record. However, it can be quite fast. If a population is reduced to a very small number, evolutionary changes can be seen over a few generations. Because micro-organisms have very short life cycles, evolutionary change in micro-organisms can be rapid. Micro-organisms can evolve resistance to a new antibiotic only a few years after the drug is first used. As a result of evolution from common ancestors, we are able to use classification of organisms to suggest evolutionary origins.

Evolutionary change and genetics The idea of continuous evolution in the living world can be traced as far back as Lucretius in the 1st century BC, but it did not gain wide acceptance until the 19th century, following the work of Scottish geologist Charles Lyell, French naturalist Jean Baptiste Lamarck, English naturalist Charles Darwin together with Alfred Russel Wallace, and English biologist Thomas Henry Huxley. Darwin assigned the major role in evolutionary change to natural selection acting on randomly occurring variations. Natural selection occurs because those individuals better adapted to their particular environments reproduce more effectively, thus contributing their characteristics to future generations. The current theory of evolution, called neo-Darwinism, combines Darwin's theory with Austrian biologist Gregor Mendel's theories on genetics and Hugo de Vries's discovery of genetic mutation.

Natural selection and chance Although neither the general concept of evolution nor the importance of natural selection is doubted by biologists, there remains dispute over other possible processes involved in evolutionary change. Besides natural selection, artificial selection, a nd sexual selection, chance may play a large part in deciding which genes become characteristic of a population, a phenomenon called ‘genetic drift’. It is now also clear that evolutionary change does not always occur at a constant rate, but that the process can have long periods of relative stability interspersed with periods of rapid change. This has led to new theories, such as punctuated equilibrium model. See also adaptive radiation; human species, origins of.

Evolution and creationism Organic evolution traces the development of simple unicellular forms to more complex forms, ultimately to the flowering plants and vertebrate animals, including humans. The Earth contains an immense diversity of living organisms: about a million different species of animals and half a million species of plants have so far been described. Some religions deny the theory of evolution, considering it conflicts with their belief that God created all things (see creationism). But most people accept that there is overwhelming evidence that the diversity of life arose by a gradual process of evolutionary divergence and not by individual acts of divine creation. There are several lines of evidence: the fossil record, the existence of similarities or homologies between different groups of organisms, genetics, embryology, and geographical distribution.

Fossils, homologies, and embryos Most organisms decompose quite rapidly after death, but sometimes a plant or animal is preserved, forming fossils – the ‘remains’ of plants or animals from many years ago, found in rocks. Fossils are usually formed when the organism is buried soon after death or occasionally by freezing, when one or more of the conditions necessary for decay are absent. Burial is generally in peat or mud, although it can also be in volcanic ash or amber – some well-preserved fossils of small animals have been found in amber. Even after burial the soft tissues of an organism may decompose so that only the skeleton of an animal or the woody parts of a plant become fossilized. Alternatively, parts of the plant or animal may be replaced by other materials as they decay – as preserved traces of animals or plants – for example, footprints, burrows, or rootlet traces. The dating of fossils presents great difficulties. In the first instance they are dated according to the stratum in which they are found and correlated with fossils in the same layer, but physical methods have been developed for estimating the absolute ages of fossils. These depend on the fact that natural radioactive carbon isotopes decay at constant rates so that the amount of radiocarbon remaining in a specimen is proportional to the length of time that has elapsed since its formation or deposition.

Although the fossil record does not actually prove the theory of evolution, a study of a series of fossils can provide a visual record of the evolution of individual species, such as the horse, and their adaptation to changing environments. Certain link fossils provide evidence of a link between species; an example of this is the Archaeopteryx, which was a birdlike animal with teeth and this fossil provides corroboration of the reptile ancestry of birds. Additional evidence for evolution is found in homologous structures. For instance, a comparison of the limb bones of several different kinds of vertebrate indicate striking similarities in their construction. Such structures are termed homologous and their existence suggests that all these animals have evolved from a common ancestor. Embryology can also provide important clues as to the ancestry of a group. The vertebrate animals all show very similar embryonic development, and all have embryonic gill slits even though the mature animal has lungs and breathes air. This is taken as evidence that the vertebrates all evolved from an aquatic ancestor breathing through gills like modern fish and the tadpole stage of amphibians. The human embryo has a small tail, the coccyx, which is vestigial (functionless) – possible evidence of a common ancestor with a tail.

Since the late 1990s, an increasing number of fully decoded genomes has enabled researchers to study evolution not only of a large number of genes, but also of higher-order genetic organization. For example, comparisons of the human genome with those of chimpanzees, mice, and rats have yielded insights into the evolution of our closest ancestors in comparison to the more distantly related mammals.

Evolutionary changes have not taken place in a linear manner, but as a branching process of descent from a common ancestor; this is often portrayed as an evolutionary tree, with humans at the top of the tree. Of the 1.5 million identifiable species existing on Earth at the present time, every one is the result of a long line of extinct species. Bacteria are among the earliest known species of life on Earth and still survive today. The human body contains ten times more bacteria cells than it does human cells.

Mechanisms of evolution Although the broad outlines of the evolutionary sequence are known, much research is still necessary to fill in the details and to discover the mechanisms of evolutionary change. Evolution depends on the presence of heritable variations in a population which confers a selective advantage on the individuals displaying them. The phrase ‘survival of the fittest’ is misleading since it implies the death of the ‘unfit’ individuals. From an evolutionary point of view, fertility is much more important than survival since if one type regularly leaves more offspring than another, the frequency of the more fertile type in the population is bound to increase. Fertility depends on many things including general vigour, the length of the reproductive period, and the ability to mate successfully.

Heritable changes arise from genetic mutations which occur spontaneously in all organisms. Many investigations that are currently being made into the genetic structures of living plant and animal populations show the relative importance of mutations and isolation in the origin of new species. It is believed that the processes now occurring on a very small scale are the same as those which have caused the evolution of the major groups over a vast period of geological time. These studies should therefore throw light on the mechanism of evolution.